The following identified two U.S. patents and one U.S. patent application are of background interest for the practice of this invention;
1. U.S. Pat. No. 3,826,865, by Quate et al, issued July 30, l974 for "Method and System for Acousto-Electric Scanning" described in the Abstract thereof the scanning of conductivity perturbations in semiconductor films by using the piezoelectric fields of acoustic surface waves. A piezoelectric substrate is situated adjacent to and spaced a small distance from a semiconductor film. A reading acoustic surface wave of relatively long pulse duration is propagated along the piezoelectric substrate in one direction and a relatively short scanning acoustic wave pulse is propagated in the opposite direction. The amplitude of the reading wave is modulated by the scanning pulse at the point where the two pass each other. An optical pattern image on the semiconductor film produces conductivity perturbations through carrier-pair generation. These conductivity perturbations appear as amplitude variations in the reading acoustic wave pulse after its interaction with the scanning acoustic wave pulse, so that the electrical output from the piezoelectric substrate contains the optical information in the pattern image on the semiconductor film. Two dimensional scanning may be accomplished by successively mechanically displacing the optical pattern being scanned with respect to the semiconductor film.
2. U.S. Pat. No. 3,826,866 by Quate et al, filed Apr. 16, 1972, and issued July 30, 1974 for "Method and System for Acousto-Electric Scanning" describes in the Abstract thereof the scanning of an energetic image to convert the information therein into an electrical signal. An electrical field is applied thereto to increase the average depletion layer width by charging the semiconductor surface states. The energetic image is impinged upon the semiconductor and begins discharging the surface states in accordance with intensity variations in the image to produce depletion layer with variations. A piezoelectric substrate is situated adjacent to the semiconductor, and a reading acoustic surface wave is propagated therein along one dimension of the semiconductor. The amplitude of the reading wave is modulated by the width perturbations of the depletion layer of the semiconductor. The output acoustic wave (ie. the modulated reading wave) is converted to an electrical signal having amplitude variations corresponding to the depletion layer width perturbations. Two dimensional scanning of the semiconductor is achieved through propagating a plurality of reading acoustic surface waves differing in frequency from each other and spaced from each other along a second dimensions of the semiconductor film.
3. Copending U.S. Pat. application Ser. No. 490,527, filed July 22, 1974, and commonly assigned, and now U.S. Pat. No. 3,919,700 issued Nov. 11, 1975 discloses a "Memory System" wherein piezoelectric photosensitive semiconductor crystal or semi-insulators are employed either to store or to process high frequency signals. Storage is accomplished in the crystal by a stable pattern of trapped electrons produced by the interference between two radiofrequency input signal pulses. The latter are applied successively to the crystal, after an initial illumination, whereby charge is trapped in shallow donor sites. The ultrasonic wave of the first pulse together with the electric field of the second cause the trapped electrons to be redistributed in a pattern which has the same spatial variation as the ultrasonic wave. In effect, the information contained in the original pulse is stored in the crystal, the latter serving as a recording medium operating over the whole radiofrequency and microwave ranges. The underlying mechanism by which charge, trapped in shallow donor sites, may be redistributed into a pattern which contains the spatial variation of an acoustic wave, is by field induced ionization from the donors into the conduction band.
The referenced copending patent application Ser. No. 490,527 presents especially relevant background information for the practice of this invention. Therefore, for purpose of disclosure herein for practice of this invention, additional substantive information presented in said application will now be paraphrased. A CdS crystal is prepared or grown so that it has high resistivity (.gtoreq. 100,000 ohm-cm) and may be doped with shallow impurities and annealed in sulfur vapor as is described in a paper entitled "Ultrasonic Amplification in Sulfur Doped CdS" by D. L. White that appeared in the Dec. 1965 issue of the Proceedings of the I.E.E.E., pp. 2157-2158.
The photosensitive semiconductor CdS has the characteristics both of low electrical conductivity and of shallow electron trapping states. These characteristics make it desirable for use as a memory when light is applied to such a crystal which excites electrons therein. Some of the excited electrons are trapped on impurities in the CdS crystal. After such light excitation, a radiofrequency pulse having frequencies in the approximate range of 10.sup.2 megacycles to 10.sup.5 megacycles is applied to the crystal so that the latter converts such pulse to an acoustic wave with the same frequency as the input radiofrequency pulse. Such acoustic wave oscillates within the CdS crystal and does not affect the separation of charges produced by the exciting light. During the lifetime of the acoustic pulse, a second radiofrequency pulse is applied to the crystal. The electric field of the ultrasonic (ie., acoustic) wave of the first pulse interacts with the electric field of the second pulse to cause the trapped electrons to redistribute into a pattern which has the same spatial variation as the ultrasonic wave. Accordingly, information, which is contained in one or both of the applied pulses, is stored in the trapped electron pattern. The stored information in the CdS crystal can be retrieved by applying thereto a third radiofrequency pulse which causes the stored electron distribution to radiate an acoustic wave in the backward direction relative to the input acoustic wave in the crystal which carried the previously stored information. The backward wave which carries the previously stored information is detected via the the piezoelectric effect at the crystal surface. The stored information can also be read out by applying a forward acoustic wave which excites the stored charge pattern to produce a microwave current which can be read out with the circuit used to apply the second microwave pulse.
In the formation of a cosine charge grating in the CdS crystal, the first pulse generates an (.omega.,k) elastic wave (i.e. phonons) via a piezoelectric effect. During the second pulse, the total electric field in the crystal is the sum of the piezoelectric field of the elastic wave generated by the first pulse and the applied electric field of the second pulse. The probability for field-induced tunneling from trapping states is a function of the magnitude of this total electric field. Therefore, it contains a term which is time independent but varies spatially as cos (k.r). Because of this tunneling probability there is an inhomogeneous trapped electronic space-charge grating which also varies as cos (k.r). The electric field of the third (reading) pulse acts on the space-charge grating to generate a backward, (as well as a forward) propagating wave which is detected at the surface of the crystal. In addition, the space-charge grating generates a uniform electric field when it is acted upon by a foward-propagating elastic wave which is piezoelectrically generated at the crystal surface by the third (reading) pulse. The output signal is the sum of these two outputs which occur simultaneously if a single microwave cavity is used to apply both the first and second pulses. In a two cavity configuration, the outputs can be separated.